As coating film-forming methods on car bodies, those widely in which adopted comprise forming multilayer coating film on-electrocoated and heat-cured coating object, by 3-coat-2-bake system application of intermediate paint→curing by baking→application of base coat paint→preheating (preliminary heating)→application of clear paint→curing by baking are successively conducted; or by 2-coat-2-bake system in which application of intermediate paint→curing by heating→application of top coat paint→baking by heating are successively conducted.
Generally the 3-coat-2-bake system is adopted when “metallic colored” coating film is to be formed using effect pigment-containing base coat paint, and the 2-coat-2-bake system is adopted in the occasions of “solid color” coating film such as of white, black or the like, using coloring pigment-containing top coat paint.
Whereas, for energy-saving, attempts are made in recent years to omit the bake-curing step after application of intermediate paint and adopt 3-coat-1-bake system successively conducting application of intermediate paint→preheating (preliminary heating)→application of base coat paint→preheating (preliminary heating)→application of clear paint→curing by baking; or 2-coat-1-bake system successively conducting application of intermediate paint→preheating (preliminary heating)→application of top coat paint→curing by baking. In particular, from the viewpoint of reducing environmental pollution by volatilization of organic solvent, 3-coat-1-bake system and 2-coat-1-bake system using water-based paints as the intermediate paint, base coat paint and top coat paint are in demand.
However, in such 3-coat-1-bake system using water-based intermediate paint and water-based base coat paint or 2-coat-1-bake system using water-based intermediate paint and water-based top coat paint, decrease in water resistance of the formed coating film due to the use of water-soluble or water-dispersible resins or deterioration in smoothness and distinctness of image of the formed coating film due to layer-mixing at the interface between the water-based intermediate paint and water-based base coat paint or that between the water-based intermediate paint and water-based top coat paint are liable to take place, which has been the problem requiring solution.
For instance, JP Hei 8 (1996)-290102A discloses that finished appearance such as gloss or distinctness of image is not impaired in the method using thermosetting water-based intermediate paint (A), thermosetting water-based base coat (B) and thermosetting clear coat paint (C), which comprises wet-on-wet system coating of the water-based intermediate paint (A) and water-based base coat (B), when the neutralization value of the base resin of the water-based intermediate paint (A) is made 10-40 mgKOH/g and that of the base resin of the water-based base coat paint (B) is made greater than the water-based paint (A) by 10-20. The multilayer coating film obtained by the coating method, however, occasionally has insufficient smoothness and water resistance.
JP 2001-205175A discloses that imbibing or inversion at the interface of coating film layers can be controlled to form laminated coating film of excellent appearance can be formed by a coating film-forming method comprising successively forming on an electrocoated substrate an intermediate coating film of a water-based intermediate paint, metallic base coating film of a water-based metallic base paint and clear coating film of a clear paint, in which the water-based intermediate paint contains aqueous dispersion of amido group-containing acrylic resin particles of a size ranging 0.01-1.0 μm, which is obtained by emulsion polymerization of amido group-containing ethylenically unsaturated monomer and other ethylenically unsaturated monomer. The multilayer film obtained by the coating film-forming method, however, occasionally has insufficient smoothness.
JP 2004-358462A discloses a process comprising (1) a step for providing electrocoated object; (2) a step for applying a water-based intermediate paint to form an intermediate coating film; (3) a step for applying on the intermediate coating film without curing the same, successively a water-based base paint and clear paint wet-on-wet, to form a base coating film and clear coating film; and (4) simultaneous baking and curing the intermediate coating film, base coating film and clear coating film, in which layer mixing between the intermediate coating film and the base coating film is effectively prevented and multilayer coating film excelling in surface smoothness can be formed when the water-based intermediate paint contains specific acrylic resin emulsion and urethane resin emulsion and the intermediate coating film formed of the intermediate paint has specific water absorption and water elution. Also the multilayer coating film obtained according to the multilayer coating film-forming process, however, there are occasions failing to produce satisfactory smoothness or distinctness of image.
International Publication WO2004/061025 Pamphlet discloses use as the intermediate paint in 3-coat-1-bake system a water-based intermediate paint composition comprising a copolymer resin emulsion and curing agent, the copolymer resin emulsion being obtained by emulsion polymerization of monomer (a) comprising at least one monomer selected from alkyl (meth)acrylates and, where necessary, at least one monomer selected from the group consisting of styrene monomer, (meth)acrylonitrile and (meth)acrylamide, acid group-containing polymerizable unsaturated monomer (b), hydroxyl group-containing polymerizable unsaturated monomer (c) and crosslinkable monomer (d), said resin having a glass transition point of −50° C.-20° C., an acid value of 2-60 mgKOH/g and hydroxyl value of 10-120 mgKOH/g. The Pamphlet status whereby curing reactability of the copolymer resin emulsion with the curing agent is raised to enable formation of multilayer coating film exhibiting good chipping resistance and water resistance and favorable finished appearance. However, there are occasions that sufficient smoothness or distinctness of image are not achieved even by the multilayer coating film obtained by this multilayer coating film-forming method.